According to the prior art, different types of pedal cranks are known, having different shapes, materials and constructive techniques in order to achieve the object of reducing as much weight as possible and, at the same time, ensuring, if not even improving, the characteristics of strength and reliability.
The trend towards manufacturing lighter components has led to the use of composite materials, in particular those formed of sheets of plastic resin material, incorporating structural fibers that contribute considerably to the structural strength of the component.
A known solution which uses the aforementioned technique is shown in published European Patent Application No. EP 1 281 609A currently assigned to Campagnolo S.r.l., wherein the body of the component is obtained by molding a thermosetting composite material inside a mold with metal inserts therein.
A suitable composite material is described in U.S. Pat. No. 4,339,490. Sheets formed of a resin matrix have differently shaped small pieces of structural fibers, of a size between 1 and 100 mm, orientated randomly into the sheet. The use of such a type of composite material for molding provides a compromise between obtaining good strength characteristics, typical of the structural fibers, and the need to keep a sufficient fluidity during molding. The size of the small pieces, allows the composite material to flow sufficiently uniformly inside the mold, creating a homogenous pedal crank body.
This molding technique, however, has some drawbacks. The molding technique used provides for arranging a predetermined amount of composite material in the central zone of the mold and, during the molding step, making such a composite material flow until the entire mold is filled, even in the end areas with the arranged metal inserts. During the molding step, the composite material flows around the metal inserts following two distinct flow paths that meet near the end of the pedal crank, completely encasing the inserts.
First, the two-pronged flow causes a disadvantageous area of discontinuity in the joint points of the two flows. Second, a disadvantage stems from the greater fluidity of the resin with respect to that of the structural fibers and due to the long route (from the center to the end of the mold), the end zones of the pedal crank have a greater percentage of resin with respect to the percentage in the rest of the pedal crank and, consequently lower strength with respect to the rest of the body. Third, a specific drawback encountered in pedal cranks obtained by molding composite material having small pieces of structural fibers is the breakage of the pedal crank in its center area. The occurrence of such a drawback is due to the fact that such a composite material with small pieces does not ensure sufficient strength in such an area of the pedal crank.
Since the ends of the pedal crank are subjected to substantial stress during pedaling, the two aforementioned drawbacks can lead to undesired breakage of the pedal crank in these areas.